1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving signals in a communication system, and in particular, to an apparatus and method for transmitting/receiving signals using a structured Low Density Parity Check (LDPC) code in a communication system.
2. Description of the Related Art
The next generation communication system has evolved into a packet service communication system, and the packet service communication system, which is a system for transmitting burst packet data to a plurality of mobile stations (MSs), has been designed to be suitable for high-capacity data transmission. It is known that the next generation communication system, together with a turbo code as a channel code, has a high performance gain during high-speed data transmissions. Additionally, the next generation communication systems are being designed to utilize the LDPC code that can increase data transmission reliability by efficiently correcting errors caused by noises generated in a transmission channel. The next generation communication system being designed that use the LDPC code includes an Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard-based communication system and an IEEE 802.11n standard-based communication system.
FIG. 1 is a diagram illustrating a structure of a signal transmission apparatus in a general communication system using an LDPC code.
Referring to FIG. 1, the signal transmission apparatus includes an encoder 111, a modulator 113, and a transmitter 115. If an information vector s that the signal transmission apparatus desires to transmit is generated, the information vector s is delivered to the encoder 111. The encoder 111 encodes the information vector s into a codeword vector c, i.e. LDPC codeword, using a predetermined coding scheme, and outputs the codeword vector c to the modulator 113. The coding scheme can be an LDPC coding scheme. The modulator 113 modulates the codeword vector c into a modulation vector m using a predetermined modulation scheme, and outputs the modulation vector m to the transmitter 115. The transmitter 115 performs transmission signal processing on the modulation vector m output from the modulator 113, and transmits the resulting signal to a signal reception apparatus via an antenna ANT.
FIG. 2 is a diagram illustrating structure of a signal reception apparatus in a general communication system using an LDPC code.
Referring to FIG. 2, the signal reception apparatus includes a receiver 211, a demodulator 213, and a decoder 215. A signal transmitted by a signal transmission apparatus is received at the signal reception apparatus via an antenna ANT, and the received signal is delivered to the receiver 211. The receiver 211 performs signal reception processing on the received signal, and outputs the reception-processed received vector r to the demodulator 213. The demodulator 213 demodulates the received vector r output from the receiver 211 into a demodulation vector x using a demodulation scheme corresponding to the modulation scheme used in the modulator 113 of the signal transmission apparatus, and outputs the demodulation vector x to the decoder 215. The decoder 215 decodes the demodulation vector x output from the demodulator 213 using a decoding scheme corresponding to the coding scheme used in the encoder 111 of the signal transmission apparatus, and finally outputs the decoded signal as a restored information vector ŝ.
The LDPC code exhibits performance characteristics that almost approach the channel capacity limit presented by Shannon's channel coding theorem. Actually, however, the communication system has difficulty in using the LDPC code to support a high data rate, because of the following restrictions.
A first restriction is the limited codeword length of the LDPC codeword. An increase in codeword length of the LDPC codeword shows the performance approaching the channel capacity limit in the Shannon's channel coding theorem, but may cause a considerable increase in the decoding complexity. Therefore, the codeword length of the LDPC codeword is limited taking the decoding complexity into account.
A second restriction is the limited maximum degree of a variable node in a Tanner graph of the LDPC code. An increase in the maximum degree of the variable node in the Tanner graph of the LDPC code causes an increase in delay in a decoding process of the LDPC codeword. That is, the maximum degree of the variable node in the Tanner graph of the LDPC code is limited taking into account the delay in the decoding process.
Therefore, there is a need for a signal transmission/reception scheme for supporting a high data rate while overcoming the restrictions for the use of the LDPC code in the communication system using the LDPC code.